Improved errors when inheriting and forgetting to call __init__

If the base class doesn't have a constructor, tell the user that it cannot be
inherited from.

Caveats:

* Doesn't work on PyPy
* Since default_object_init is inline, if the default instance base was no longer
  a PyHeapTypeObject it could crash.
* If pybind11_object_init changed in the future, and multiple pybind11 modules are
  present, get_internals might return an old version of the function

include/pybind11/detail/class.h

@@ -171,8 +171,13 @@ extern "C" inline PyObject *pybind11_meta_call(PyObject *type, PyObject *args, P
     // Ensure that the base __init__ function(s) were called
     for (const auto &vh : values_and_holders(instance)) {
         if (!vh.holder_constructed()) {
-            PyErr_Format(PyExc_TypeError, "%.200s.__init__() must be called when overriding __init__",
-                         vh.type->type->tp_name);
+            auto message = "%.200s.__init__() must be called when overriding __init__";
+#if !defined(PYPY_VERSION)
+            if (vh.type->type->tp_init == get_internals().default_object_init()) {
+                message = "%.200s has no __init__ and cannot be used as a base class from Python";
+            }
+#endif
+            PyErr_Format(PyExc_TypeError, message, vh.type->type->tp_name);
             Py_DECREF(self);
             return nullptr;
         }
@@ -302,6 +307,8 @@ extern "C" inline PyObject *pybind11_object_new(PyTypeObject *type, PyObject *,
 /// An `__init__` function constructs the C++ object. Users should provide at least one
 /// of these using `py::init` or directly with `.def(__init__, ...)`. Otherwise, the
 /// following default function will be used which simply throws an exception.
+///
+/// Use `get_internals().default_object_init()` instead of using this function directly
 extern "C" inline int pybind11_object_init(PyObject *self, PyObject *, PyObject *) {
     PyTypeObject *type = Py_TYPE(self);
     std::string msg;
@@ -620,7 +627,7 @@ inline PyObject* make_new_python_type(const type_record &rec) {
         type->tp_bases = bases.release().ptr();
 
     /* Don't inherit base __init__ */
-    type->tp_init = pybind11_object_init;
+    type->tp_init = internals.default_object_init();
 
     /* Supported protocols */
     type->tp_as_number = &heap_type->as_number;

include/pybind11/detail/internals.h

@@ -121,6 +121,11 @@ struct internals {
         PYBIND11_TLS_FREE(tstate);
     }
 #endif
+
+    inline initproc default_object_init() {
+        auto heap_type = (PyHeapTypeObject*)instance_base;
+        return heap_type->ht_type.tp_init;
+    }
 };
 
 /// Additional type information which does not fit into the PyTypeObject.

tests/test_class.py

@@ -132,6 +132,21 @@ def __init__(self):
         expected = "m.class_.Hamster.__init__() must be called when overriding __init__"
     assert msg(exc_info.value) == expected
 
+    # Base doesn't have __init__
+    class ChChimera(m.Chimera):
+        def __init__(self):
+            pass
+
+    with pytest.raises(TypeError) as exc_info:
+        ChChimera()
+    if env.PYPY:
+        # can't detect no __init__ in PyPy
+        expected = "Chimera.__init__() must be called when overriding __init__"
+    else:
+        expected = \
+            "m.class_.Chimera has no __init__ and cannot be used as a base class from Python"
+    assert msg(exc_info.value) == expected
+
 
 def test_automatic_upcasting():
     assert type(m.return_class_1()).__name__ == "DerivedClass1"